Multiple -port optical package and DWDM module

ABSTRACT

Optical package and module designs use multiple-port (e.g. six-port) optical packages to create compact DWDM modules, add/drop packages, heat dissipation packages, optical amplifier filters, and the like.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/599,168, filed on Jun. 22, 2000, entitled “THREE-PORT FILTERAND METHOD OF MANUFACTURE,” by Scott M. Hellman et at., now U.S. Pat.No. 6,343,166, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical telecommunication systems and,in particular, to an apparatus and method of manufacturing opticaldevices employed in such telecommunication systems.

2. Technical Background

Up to three port filtering and isolating packages are widely used inlocal and long distance optical telecommunication networks. Thesenetworks comprise various spectral shaping and isolating opticalassemblies as parts of dense wavelength division multiplexing (DWDM)systems. The necessity to design reliable optical devices for suchsystems, which are subject to various thermal and mechanical loadsduring their 20 to 25 year lifetime, is of significant importance. Atypical example of such optical devices is an optical filter assembly. Atypical optical filter assembly comprises two (input and reflective)optical glass fibers inserted into a dual-capillary ferrule to produce afiber-ferrule sub-assembly, a GRIN lens, and a filter. The opticalcomponents of the filter assembly are embedded into an insulating glasstube, which in turn is mechanically protected by a metal housing. In atypical 3-port package the above dual-fiber filtering assembly iscombined with an output collimating assembly leading to a single opticalfiber. These filter assemblies typically exhibit insertion losses higherthan desired, resulting in degraded overall performance of thecommunications system or module. The problem is particularly acuteduring exposure to ambient operating conditions where temperature isvariable.

Typical input glass ferrules employ one of two designs. A singlecapillary suitable for containing multiple glass fibers or separatecircular capillaries for each fiber have been used, each with relativelyshort (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such inputferrules, the optical fiber is subjected to an S-bending over the shortconical end portion which typically exceeds 50% of the fiber diameter(for a fiber having a 125 μm diameter) on a span of about 6 to 10diameters in length. This excessive micro bending increases theinsertion losses. Although the multi-capillary design reduces thelateral deflection of fiber interconnects compared to the ellipticalsingle-capillary design, the short length of the cone end of suchferrules cannot reduce the micro bending of the fiber and its inherentinsertion loss. Fiber-ferrule subassemblies employing such ferrules aremanufactured by inserting the optical fibers stripped of their polymercoating into the respective ferrule capillaries; epoxy bonding thefibers into the ferrule capillaries, including the conical end portions;grinding and polishing an angled facet on the fiber-ferrule; anddepositing on the polished surface an anti-reflection (AR) coating. Oncefinished, the fiber-ferrule is aligned and assembled with thecollimating GRIN lens and then embedded into the insulating glass tube,which, in turn, is protected by a metal housing.

There are two different technical solutions used in the design of bondssecuring the components of an optical assembly. A low compliance bondbetween thermally well matched glass fibers and the glass ferrule is anapproach commonly used by some manufacturers. The adhesives used areheat-curable epoxies with high Young's modulus (E>100,000 psi) andmoderate to high thermal expansion coefficients (α=40-60 10⁻⁶° C.⁻¹). Atypical example would be 353 ND EPO-TEK epoxy adhesive. In addition, thebond thickness used is very small.

Silicon adhesives are used to bond thermally mismatched glass tubes withmetal housings and glass optical elements with metal holders. In thesejoints, a high compliance design is used. The silicones, which can becured between 20-150° C. in the presence of moisture, are typicallycharacterized by an extremely low Young's modulus (E<500 psi) and highthermal expansion (α=180-250 10⁻⁶° C.⁻¹). A typical example would be DC577 silicone, which can be used to bond, for example, a metal opticalfilter holder to a GRIN lens.

Adhesive bonding with subsequent soldering or welding is used toencapsulate a filtering assembly into a three-port package of a DWDMmodule. A precise alignment achieved during initial assembly of a filterprior to final packaging can be easily decreased due to the adhesivecuring process and the high temperature thermal cycles associated withsoldering or welding during the final packaging of the components. Suchmanufacturing processes and resulting components have several problemsresulting from stresses on the optical components due to the thermalcontraction mismatch between the glass and metal materials,polymerization shrinkage in adhesive bonds, and structural constraintsinduced by bonding and final soldering during encapsulation. Thesestresses lead to displacements of optical components during bonding andsoldering, resulting in 0.3 to 1 dB or greater increases in theinsertion loss.

Such a filter package enclosure, which is typically formed of six toeight concentric protective units, has micron transverse tolerances.Maintaining these tolerances requires precision machining,time-consuming alignment, and soldering with frequent rework. As aresult of these limitations, the optical performance specifications arelowered and cost is increased. As an example, soldering typicallyincludes several re-flow cycles. This induces local thermal stresses inthe nearby adhesive bonds and leads to the degradation of the polymeradhesive, resulting in repositioning of optical components and a shiftin the filter spectral performance. With such design, soldering may alsoresult in the contamination of optical components through direct contactwith molten solder and/or flux.

However, for many applications, it is desirable to obtain a highaccuracy thermally compensated optical multiple-port package that can berelatively inexpensive, reliable, and have a low insertion loss.Additionally, a package design should be adequate not only tomechanically protect the fragile optical components, but also tocompensate for and minimize the thermally induced shift in spectralperformance. Further, it is desirable to obtain a multiple-port package,such as six port packages, with the same qualities since they furtherreduce costs, reduce size, and also result in reduced insertion loss.Thus, there exists a need for such optical packages and a process formanufacturing such optical packages, which is miniaturized, has a lowinsertion loss, is inexpensive to manufacture, and which results in adevice having reliable, long-term operation.

SUMMARY OF THE INVENTION

The present invention provides an improved optical assembly (e.g.,optical filter assembly) with a low insertion loss (IL) and provides anassembly of the optical components, such as input ferrules, collimatinglenses, and filters, utilizing bonding adhesives in a manner whichallows the alignment of the individual components relative to oneanother with a precision and a manufacturability that makes it possibleto produce commercial devices having five, six or more ports. Thisheretofore had not been achieved. In one aspect, the invention includesan improved input ferrule and filter holder which permits activealignment and bonding through the utilization of UV and thermallycurable adhesives and improved thermal curing to greatly reduce relevantinternal stresses in the subassembly so formed. For assemblies havingmultiple pairs of fibers (e.g., five or more port devices), theinvention also provides improved fiber ferrule designs, alignmentmethods, and methods to permit the manufacture of devices that have lowIL, operate over a wide temperature range, are reliable, and costeffective.

In one aspect of the invention, improvements to fiber ferrules areprovided including capillary designs and tolerances. The inventionprovides designs for capillaries which resist movement of the opticalfibers during adhesive curing, soldering, welding, and environmentalthermal changes. One technique uses washers to precisely positionoptical fibers in a capillary. Yet another aspect of the invention isthe selection of optical fibers based on geometric properties such as:outer (cladding) diameter, circularity of the cladding (ovality), andcore concentricity. In another aspect, the invention teaches matchingthe separation distance (SD) between optical fibers and the relationshipto angle of incidence (AOI) of the optical filter. Tolerances for theseparation distance are provided which make possible the commercialmanufacturability of multiple-port devices with five, six or more ports.The optical alignment process becomes more critical and complex as thenumber of ports increases and therefore the invention provides methodsfor handling this more complex alignment. A method of selecting anoutput collimating assembly is also provided.

Methods embodying the present invention include the steps of activelyaligning a filter holder and filter to a collimator assembly including aGRIN, aspheric, or other collimating lens mounted thereto, axiallyseparating the filter holder and lens in a movable fixture, placing a UVand thermally curable adhesive on the periphery of the lens, moving thelens into engagement with the filter holder having a filter mountedtherein, aligning the collimator assembly with respect to the filterholder while monitoring the input and reflected signals of the opticalfibers coupled to the lens for insertion loss less than about 0.2 dB,and applying UV radiation through the filter end of the filter holder toinitially cure the aligned subassembly. In an embodiment of theinvention, the subassembly is subsequently thermally cured through anaccelerated dark cure sequence followed by a final high temperaturecuring. In another embodiment of the invention, UV radiation is appliedto the filter holder/lens interface through one or more apertures formedin the side of the filter holder which overlaps the lens. The UV lightsource may be dithered such that UV radiation uniformly covers thecylindrical interface between the filter holder and the outer surface ofthe lens. In yet another embodiment of the invention, the filter andlens are pre-aligned prior to the application of adhesive by monitoringthe input and reflected signals of the fibers while adjusting the X-Ypositioning for a maximum detected signal.

In a preferred method of manufacturing the invention, subsequent to theUV curing process, the assembly is cured through a stress relaxationcycle at about 40-50° C. for two to four hours followed by a thermalcuring cycle of about 95-100° C. for one to two hours.

In one embodiment of the invention, an input ferrule is employed with aninput cone having an axial length greater than about 2.5 mm to reduceS-bending of input fiber, thereby minimizing resultant insertion losses.In another embodiment of the invention, a generally cylindrical filterholder having an annular seat formed in one end for receiving a filterand a lens-receiving aperture at an opposite end having an internaldimension which allows micro-tilting of the filter holder relative tothe lens is used to provide an alignment of the filter at an angle ofless than about 1° to the axis of the lens. The preferred filter holderincludes slots or openings in the lateral surface such that UV lightenters and cures adhesive between the lens and filter holder. An opticalfilter assembly of a preferred embodiment of the present inventionincludes such an improved ferrule and/or a filter holder coupled inalignment with one another in a suitable housing.

The methods and apparatus described herein facilitate the manufacture ofa multiple-port optical device which results in several advantages. Forexample, in a six port device having two pair of optical fibers in theinput collimating assembly, one filter operates with at least twotransmitted light beams and splits the beams into at least two reflectedand two transmitted beams, thereby reducing by half the number ofoptical filters, collimating lenses and enclosure units. Thus, forexample, the same six-port filtering package can be used in themultiplexing and de-multiplexing operations of a DWDM moduleincorporating concatenated six-port packages. A typical DWDM moduleincludes from two to eight six-port packages. In this case, the numberof filter chips, collimating lenses, and fiber ferrules will be reducedby one-half compared to using three port packages.

The manufacturing method and optical element assembly of the presentinvention, therefore, provides an improved performance optical assemblyutilizing a unique input ferrule, filter holder, and an assembly methodfor providing a low cost, highly reliable, and improved performanceoptical element assembly, such as a three-port collimating filter orsix-port collimating filter assemblies, and using these assemblies inDWDM modules which can be used in an optical communications system.

The devices of the instant invention are applicable for both single modeoptical fibers that are applicable to DWDM operations and forpolarization maintaining fibers that can be used in the crystal basedisolators, circulators, polarization splitters, and the like.

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the description which follows together withthe claims and appended drawings.

It is to be understood that the foregoing description is exemplary ofthe invention only and is intended to provide an overview for theunderstanding of the nature and character of the invention as it isdefined by the claims. The accompanying drawings are included to providea further understanding of the invention and are incorporated andconstitute part of this specification. The drawings illustrate variousfeatures and embodiments of the invention which, together with theirdescription serve to explain the principals and operation of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a filter subassembly embodying thepresent invention;

FIG. 2 is a partial vertical cross-sectional schematic view of thesubassembly shown in FIG. 1;

FIG. 3 is a vertical cross-sectional schematic view of a three-portfilter assembly embodying the present invention;

FIGS. 4 and 4A are an enlarged vertical cross-sectional and right endview, respectively, of a prior art ferrule employed in a prior artfilter assembly;

FIGS. 5 and 5A are an enlarged vertical cross-sectional view and rightend view, respectively, of a ferrule employed in the filter subassemblyof FIGS. 1 and 2 and the filter of FIG. 3;

FIG. 6 is an enlarged vertical cross-sectional schematic view of animproved filter holder of the present invention also illustrating itsmethod of assembly;

FIG. 7 is a schematic view illustrating the frontal polymerization of aUV or thermally curable bonding adhesive when UV light is propagatedtransversely through a filter, as illustrated in FIG. 6;

FIG. 8 illustrates the spectrum of a mercury light source showing asignificant portion of the UV light spectrum;

FIG. 9 is the measured UV-transmission spectrum of a commerciallyavailable thin film filter used in the structure shown in FIGS. 1, 2, 3,and 6;

FIG. 10 is a graph of the accelerated dark cure and thermal cure of thesubassembly shown in FIG. 6;

FIG. 11 is a perspective view of an alternative embodiment of a filterholder embodying one aspect of the present invention;

FIG. 12 is a vertical cross-sectional schematic view of a three-portfilter employing the filter holder shown in FIG. 11;

FIG. 13A is a cross-sectional view of a fiber-ferrule assemblyillustrating a rounded square capillary;

FIG. 13B is a cross-sectional view of a fiber-ferrule assemblyillustrating a dual-oval capillary;

FIG. 13C is a cross-sectional view of a fiber-ferrule assemblyillustrating a four-circular capillary;

FIG. 13D is a cross-sectional view of a six-fiber ferrule having arectangular capillary;

FIG. 13E is a cross-sectional view of a fiber-ferrule assemblyillustrating capillaries formed by symmetrical grooves formed in dualsilicon wafers;

FIG. 13F is another embodiment of a fiber-ferrule formed from twowafers;

FIG. 13G is a cross-sectional schematic view of a finished two-waferferrule inside a glass sleeve;

FIG. 13H illustrates the preferred V-groove and alignment rodconfiguration;

FIG. 13I is a cross-sectional view of a fiber ferrule illustratingalignment of fibers with two wafers and application of liquid adhesive;

FIG. 13J is a cross-sectional view of a fiber ferrule having arectangular capillary for variable separation distance;

FIG. 13K is a cross-sectional view of a fiber ferrule having dualrectangular capillaries for variable separation distance;

FIG. 13L is a cross-sectional view of a fiber ferrule having elongateddual rectangular capillaries;

FIG. 13M is a cross-sectional view of a fiber ferrule having dual ovalcapillaries;

FIG. 13N is a cross-sectional view of a fiber ferrule having threecapillaries;

FIG. 14A is a view of an alignment washer;

FIG. 14B is a cross-sectional exploded view of a fiber-ferrule assemblyusing alignment washers;

FIG. 15 is an exemplary table for matching optical fiber SD and filterAOI.

FIG. 16A is a schematic diagram of a four-port filter assembly;

FIG. 16B is a schematic diagram of a four-port filter assembly coupledto an amplifier;

FIG. 16C is a schematic diagram of a four-port filter assembly coupledto two amplifiers;

FIG. 16D is a schematic diagram of five-port filter package;

FIG. 16E is a schematic diagram of a six-port filter package coupled towaste energy terminals;

FIG. 16F is an alternate schematic diagram of a six-port filter packagecoupled to waste energy terminals;

FIG. 16G is a schematic diagram of a six-port add/drop package;

FIG. 16H is a schematic diagram of an eight-port optical package;

FIG. 16I is a schematic diagram of an eight-port add/drop package; and

FIG. 17 is a schematic diagram of concatenated six-port packages to forma DWDM module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIGS. 1 and 2, a brief description of an opticalelement (e.g., filter) subassembly 10 is first presented. The inventionis described and illustrated using an exemplary three-port filterdevice, however, the invention also applies to multiple-port devicessuch as six-port devices. For multiple-port devices, the number andposition of fibers in ferrule 16 changes accordingly. The dual fibercollimating and filtering subassembly 10 includes an outer cylindricalmetal housing 12, which is bonded at 13 (FIG. 1) around input andreflection optical fibers 18 and 20, respectively. Housing 12 surroundsan insulating cylindrical boro-silicate or fused silica sleeve 14 (FIG.2) within which there is mounted a dual capillary glass ferrule 16receiving an input optical fiber 18 and a reflective optical fiber 20.The ends of fibers 18 and 20 in ferrule 16 face a collimating lens 22,such as, for example, a GRIN lens, which has polished facets on theinput end, and (as seen in FIG. 2) which face and align with the ends ofoptical fibers 18 and 20 held in place by ferrule 16. Lens 22 collimateslight from input fiber 18 into parallel rays, transmitting them to anoptical element which may be a thin film filter 24, a birefringentcrystal, or other appropriate optical element. The end of thecollimating lens 22 that is closest to the filter 24 is referred to asthe output end of collimating lens 22. A filter holder 26 is mounted tothe end 21 of the collimating lens 22 according to the method of thepresent invention and includes an axial aperture 27 allowing light fromlens 22 to impinge upon filter 24 and the reflective light to bedirected to reflective optical fiber 20. Filter holder 26 also securesfilter or crystal 24 in alignment with the collimating lens 22 withaperture 27 extending between the filter 24 and lens 22. Thefiber-ferrule 16, lens 22, and insulating sleeve 14 are collectivelyreferred to as an input collimating assembly 35. Collimating assembly 35may also include cylindrical metal housing 12. A similar single fibercollimating assembly structure is collectively referred to as an outputcollimating assembly 35′ and is shown in FIG. 3.

Before describing the manufacture of the subassembly 10 forming a partof an overall three-port filter, a three-port filter 30 is brieflydescribed. FIG. 3 is also representative of a multiple-port device,however, for a multiple-port device, the number and position of fibersin ferrules 16 and 39 changes accordingly. As shown in FIG. 3,three-port filter 30 includes an outer cylindrical metal sleeve 32 intowhich subassembly 10 is mounted and secured by a cylindrical interfaceof solder and/or welding material 31 applied to the solder joint as seenin the schematic diagram of FIG. 3. Solder and/or weld material 31 maybe applied through suitable apertures 32A in metal sleeve 32. The outputsignal from filter 24 is received by an aligned collimating output lens34 similarly secured within a boro-silicate or fused silica glass sleeve36 surrounded by a metal sleeve 37 which, in turn, is mounted within theinterior of outer protective sleeve 32 utilizing a cylindrical solderinterface 33. The output lens 34, ferrule 39, glass sleeve 36, and metalsleeve 37 form the output collimating assembly 35′. An output opticalfiber 38 couples to the desired wavelength output signal from three-portfilter 30 to the communication linkin which the three-port filter 30 isinstalled. Thus, for example, the three-port filter 30 may be employedto receive a plurality of wavelengths from input optical fiber 18, passa single output wavelength to output fiber 38, and return the remainingsignal wavelengths to reflective optical fiber 20. The method ofassembling subassembly 10 and its structural elements are unique anddescribed in detail below. Further, the specific method of aligningoutput collimating assembly 35′ within sleeve 32 will also be describedbelow.

One problem associated with prior art ferrules is illustrated by FIG. 4showing a vertical schematic cross-sectional view of a prior art inputferrule 40. Ferrule 40 is made of a conventional glass material such asfused silica or boro-silicate glass and includes a pair of spaced-apartcapillaries 42 and 44 having a diameter sufficient to receive thestripped input and reflective optical fibers 18 and 20 having a diameterof about 125 μm. The overall diameter, however, of optical fibers 18 and20 includes a protective polymeric sheath and is approximately 250 μm.Optical fibers 18 and 20 are cemented within the conical input section46 of the prior art ferrule 40 utilizing a thermally curable epoxyadhesive providing a strain-relief connection of the coated fibers 18and 20 within the glass ferrule. As the stripped optical fibers 18 and20 exit the polymeric sheath and enter the capillary tubes 42 and 44over the length of 1.2 mm of the conical input section 46, they are bentat area 47 schematically shown in FIG. 4. This S-bending of the opticalfibers interconnection to the ferrule 40 results in deflection of thefiber, which exceeds 50% of the fiber diameter. This induced microbending of the fiber increases insertion loss of the signals applied tothe lens 22 due to the geometry of ferrule 40.

Capillaries 42 and 44 of ferrule 40 are spaced apart a distance “D 1,”as shown in FIG. 4A that with the coned length provided by prior artferrules as shown in FIG. 4, results in such excessive micro-bending ofthe optical fibers and resultant insertion losses. The alternate ferruleconstruction in which a single elliptical capillary is provided forreceiving adjacent optical fibers and having a similar input coneconstruction suffers even more from the bending problem. In order togreatly reduce the insertion loss due to the undesirable S-bending ofinput fibers, an improved ferrule 16 of the present invention, whichforms part of the subassembly 10 as seen in FIGS. 1 and 2, is employedand is described in FIGS. 5 and 5A.

In FIG. 5, a ferrule 16 is shown which has an input cone 17 with anaxial length in the preferred embodiment in excess of 2 mm andpreferably 2.4 mm or approximately twice the length of prior art inputcones. The input diameter “D2” of input cone 17 is approximately 0.8 mmto accommodate the 500 mm combined diameter of input fibers 18 andreflective fiber 20 and allow room for epoxy to bond the fibers withincone 17. The exit diameter “D3” of cone 17 adjacent capillaries 19 and21, which receive and secure the optical fibers 18 and 20 therein, ispreferably determined as:

D3=2f _(d) +D1

or

D3=250 μm+D1

where f_(d) is the fiber diameter with the sheath material removed

This accommodates any spacing D1 between the fibers and the 125 μmdiameters of each of the stripped input and reflective fibers, allowingalso approximately a one μm gap at the input to capillary tubes 19 and21 for epoxy to securely seat the input and reflective fibers withinferrule 16. To obtain the best possible performance, the fibers shouldbe selected for their geometric properties. Three important propertiesand the preferred tolerances are outer cladding diameter of 125 μm+/−0.2μm, non-circularity of the cladding less than 0.2%, and core to claddingconcentricity is less than 0.2 μm. By expanding the axial length “L” ofcone 17 to nearly twice that of prior art input ferrules, S-bending issubstantially avoided, providing substantially a nearly equal opticalpath length for both the input and reflective fibers and reducinginsertion losses. This technique is also applicable to ferrules havingmore than two optical fibers and to ferrules with single or multiplecapillaries.

The fibers are epoxied within the ferrule 16 with an epoxy adhesive suchas, for example, 353 ND EPO-TEK epoxy adhesive available from EpoxyTechnology, Billerica, Mass., and cured at about 110° C. for one andone-half hours. It is preferable to post-cure the assembly at 125-130°C. for one-half hour to reduce moisture absorption. The end-face 28 ofthe ferrule with inserted and bonded optical fibers is ground andpolished to produce approximately 8° angle elliptical facet to the axisof the ferrule. Ferrule 16 is then cemented within the surroundingthermally insulating glass sleeve 14 (FIG. 2) to form input collimatingassembly 35. Prior to the insertion of the ferrule 16 into sleeve 14,the lens 22 has been installed and cemented in place. The ferrule isaligned with a gap “G” (FIG. 2) of about 1 to 1.5 μm between the ends ofthe lens 22 and the ferrule to allow the axial and rotational activealignment of the ferrule to the lens 22 by rotating the ferrule withinsleeve 14 and axially positioning it to accommodate the surface angle ofthe lens 22, which may run between 7.8° to 8.1°. For a three-portassembly, a signal is applied to the input fiber 18 while monitoring theoutput of the GRIN lens within sleeve 14. For a multiple-port assembly,such as used in a six-port device, the alignment process is similar;however, signals are applied to each of the input fibers and the ferruleis axially and rotationally positioned to optimize the alignment for allof the signals. This assures the minimum insertion loss and maximumsignal coupling between the optical fibers and the input collimatinglens 22, which subsequently receives the filter holder and filtertherein as now described in connection with FIG. 6.

Referring now to FIG. 6, the subsequent positioning of filter 24 andfilter holder 26 onto end 21 of the lens 22 is described. Matching theAOI of the filter 24 with the separation distance (SD) of the fibers 18and 20 is important. A filter 24 with a desired AOI is selected for usein the assembly 10. An input collimating assembly 10 is selected havinga ferrule 16 which has an SD that corresponds to the AOI of filter 24.The SD is accurately measured, preferably within 0.5 μm, and the filterholder 26 is mounted on the selected input collimator assembly 35. Thematching process in the case of the four-fiber ferrule, used in six-portdevices, is preferably performed as follows. The SD of one of the pairsof fibers is matched to the filter AOI. The alignment match for thesecond pair of fibers is provided automatically when the structuraltolerances described above for the capillaries and fibers have beensatisfied. Therefore, it is important for the SD for each pair of fibersto be approximately equal. Preferably the SD tolerance for each pair offibers is within 0.5 μm. The tolerances are further discussed below indiscussion of FIGS. 14 and 15.

Filter holder 26 has a cylindrical aperture 25 at its lower end, as seenin FIG. 6, which overlies the cylindrical diameter of lens 22. Thediameter of the aperture 25 is large enough to provide a gap “G1” ofabout 50 μm surrounding output end 21 of collimating lens 22. This, asdescribed below, allows the micro-tilting of the filter holder 26 withrespect to lens 22 for precisely aligning the filter 24 and lens 22while accommodating the bonding adhesive employed for securing thefilter holder to the lens 22. Holder 26 is made of a material which hasa coefficient of thermal expansion which is close to that of the lensand, in a preferred embodiment of the invention, is a unit made ofSS17-4-PH stainless steel. Prior to assembling of filter holder 26 tolens 22, the filter 24 is mounted within the filter holder 26, which hasa cylindrical aperture 29 with a seat 50 canted at an angle ∝₁ (FIG. 6)of approximately 1.5° to 2° and preferably about 1.8° to accommodate theapproximate 0.3°to 0.7° angular discrepancy between the front and rearsurfaces of a typical filter chip 24. The cant of seat 50 also has thefavorable effect of reducing the tilt angle of holder 26 relative tolens 22. The filter 24 is secured within cylindrical aperture 29utilizing conventional epoxy or even silicone bonding adhesives, such asDC577 or CV3 2000, and the filter chip 24 can be any commerciallyavailable thin-film filter. In the illustrated embodiment, acommercially available filter having dimension of, for example, 1.4 by1.4 by 1.5 mm is used. Such filters are available commercially fromCorning Incorporated. The assembly and met hods of the invention canalso be used with other optical devices in place of filter 24, such asvarious crystal-based components.

With filter 24 in place in filter holder 26, the holder is clamped in avertically (as seen in FIG. 6) movable clamp which can also be rotatedsuch that filter holder 26 can be moved into and out of engagement withlens 22 as well as rotated and tilted for actively aligning the opticalaxis of the filter to the lateral surface of the lens 22 to minimizeinsertion loss. Active alignment is the process of aligning the opticalelements while applying input light signals to the device and monitoringan output signal. This is in contrast to passive alignment which is theprocess of aligning optical elements in the absence of a light signal.

The active alignment in an embodiment of the invention is achieved, forexample, by applying a signal at about 1530 nm to input fiber 18 (FIGS.1-3) while monitoring the reflected signal on fiber 20. Filter holder 26is then micro-tilted in orthogonal directions and also rotated inincrements of about 2° to 5° as necessary to achieve minimum insertionloss as determined by monitoring the input and reflected signals. Thereare six degrees of freedom in which the holder 26 may be moved relativeto lens 22. These include microtilting on the XZ plane and YZ plane ofFIG. 6, rotating about the Z axis, moving lateral along the X and Yaxis, and raising and lowering the holder along the Z axis. Generally,only rotation and micro tilting along the XZ and YZ plane are sufficientto align the elements.

The preferred embodiment uses an automated iterative process in whichthe IL for each pair of fibers is monitored for each tilt or rotation.The iterative process repetitively adjusts the filter holder andmonitors the input and output signals and eventually locates an optimumalignment as defined by predetermined tolerances. The alignment processincreases in complexity with increasing pairs of optical fibers inmultiple-port systems. The preferred method of alignment comprises thesteps of aligning each pair of fibers separately and then selecting anaverage alignment position or a median position. For six-port devices,the optimum alignment achieved for the first pair of the reflective andinput fibers can be slightly lowered when aligning the second pair ofthe reflective and input fibers. Also, in the case of six-port devices,the iterative process has been found to be unexpectedly short (i.e. fewiterations) because of the tolerances selected in accordance with anembodiment of the invention. When a first pair of fibers is opticallyaligned, the second pair of fibers may be close to alignment since thesecond pair of fibers have virtually the same separation distance as thefirst pair of fibers.

During this alignment process, lens 22 and its sleeve 12 are mounted inan XYZ micro-adjustable stage of conventional construction to hold theprojecting end of lens 22 in cavity 25 of holder 26. Once the optimumangular position of the filter holder 26 to lens 22 is determined, thefilter holder 26 is raised axially away from the lens (while maintainingthe angular relationship) to allow access to the side wall of lens 22.While separated, preferably four or more drops of bonding adhesive arepositioned on the outer peripheral circumferential surface of the end 21of lens 22, with care being taken not to touch drops of the epoxyadhesive to the lens end face surface. The filter holder 26 is thenlowered over the lens 22, wiping the adhesive in the annular spacebetween cavity 25 and lens 22. Next, the XZ axis of the stage may befurther adjusted while monitoring signals applied to the input andreflective optical fibers 18 and 20 to assure a minimum insertion loss.Similarly, the YZ axis of the stage may also be adjusted whilemonitoring the signals to assure proper alignment and a minimumreflected insertion loss of no greater than about 0.3 dB. A variety ofUV and thermally curable epoxies were tested, and it was determined thatthe bonding adhesive which worked unexpectedly well was commerciallyavailable EMI-3410, which is a UV and thermally curable filled adhesiveavailable from Electronic Materials, Inc., of Breckenridge, Colo.

By providing a gap of approximately 50 μm between the inner surface ofcylindrical aperture 25 of filter holder 26 and the outer diameter oflens 22, the optical axis of the lens can be precisely aligned with theoptical axis of filter 24. Filter holder 26 is adjustable within anangle ∝₂ of less than about 1.0°, as shown in FIG. 6. This activealignment of the lens 22 and filter holder 26 is achieved by themovement of the lens 22 in the XZ and YZ planes, as shown in FIG. 6,utilizing a standard micro-stage (i.e., micropositioner). In oneembodiment of the invention, one or more sources of ultra violetradiation such as sources 60 and 61 are employed to expose the bondingadhesive at the interface between holder 26 and lens 22 to ultravioletradiation to cure the bonding adhesive sufficiently such that thedesired relationship between the lens 22 and filter 24 is fixed untilthe adhesive is finally thermally cured.

As seen by the diagram of FIG. 7, by injecting ultra violet radiationfrom source 60 into the exposed end of filter 24, ultra violet radiation(indicated as 63) is dispersed as the UV radiation propagatestransversely through the filter and into the adhesive layer 55 (FIG. 6),causing frontal polymerization of the adhesive due to UV lightpropagating through the filter. In most instances, the UV radiation 63from source 60 through filter 24 will, upon an exposure of about 20seconds at a distance of about 2.5 cm between the source and the filter24, result in sufficient UV curing of the adhesive to fix the filterholder to the lens 22. In addition to exposing the adhesive 55 throughfilter 24 utilizing a UV light source 60, an additional UV light source61 can be employed to direct UV radiation 63 through the gap G2 betweenthe lower annular end of filter holder 26 and the top annular surface ofsleeve 12 with 40 second exposures for a total exposure of about 100seconds of UV radiation to cure the adhesive in the annular area of gapG1 at the lower end of filter holder 26. After the UV curing, whichtends to temporarily induce stresses typically of from 200 to 300 psi orhigher in the subassembly, thermal cure stress release and curing isprovided as described below. Before such curing, however, input andoutput signals are monitored to assure that the reflected insertion loss(IL) remains less than about 0.3 dB and thermal change in IL is belowabout 0.05 dB. The UV from light source 61 can be rotated around theperiphery of the subassembly during successive exposures. The UV lightcan be delivered also through slots or openings formed into the lateralsides of the filter holder 22 as described below.

The UV sources 60 and 61 have spectral emissions, as illustrated in FIG.8, which shows the spectrum of a mercury light source. FIG. 9illustrates the experimentally determined UV transmission spectrum ofsuch a light source through a bulk filter chip of the kind used in thefilter 24 illustrated in FIG. 6. The convolution of these spectraindicates that a sufficient portion of the UV light spectrum propagatesto the bond layer through the filter 24 and that the duration of the UVcure cycle results in a nearly zero change of insertion loss over aperiod from 630 to 700 seconds. The UV initiated cure induces initialstresses due to polymerization shrinkage. For a typically highly filledepoxy adhesive with a limited volume of shrinkage (on the order of0.2%), the induced stress can be as high as 300 to 600 psi. The stressesinduced by the UV curing, which fix the alignment of the filter to thecollimating lens 22, are relieved and the bonding adhesive 55 furthercured during thermal curing of the subassembly 10 in a conventional ovenwhich is controlled to provide the stress relaxation and thermal curecycles as illustrated in FIG. 10.

The graph of FIG. 10 illustrates an accelerated and thermally assistedstress relaxation phase in an oven which is controlled to provideseveral short thermal cycles at an elevated temperature preferably notexceeding 50% of the minimum temperature of thermal cure. The cycletypically starts at room temperature, and the temperature is increasedto cycle between about 40° and about 60° C. over ten to fifteen cyclesper hour for a total period of approximately one and one-half to fourhours. The thermal cycling induces the variable mismatch stresses in theglass, metal filter holder, and the adhesive. Although the rate ofstress relaxation in the adhesive increases with an increase in themismatch stresses, this stress level is limited by the allowable elasticlimits. These cyclic changes in temperature induce the creep in adhesivethat leads to the additionally accelerated stress relaxation. By cyclingthe temperature as shown in FIG. 10, the typically 12 to 24 hour roomtemperature dark cure is reduced to about one to two hours. In thiscase, any thermally induced repositioning of optical components (e.g.filters) is drastically reduced.

As seen in FIG. 10 after the thermally assisted stress relaxation phase(TASR), the assembly is subjected to a final thermal cure for about twoto about two and one-half hours at a temperature of from about 85° toabout 100° in the case of the preferred EMI-3410 adhesive. By utilizingthe thermal curing cycle illustrated in FIG. 10, the elevatedtemperature induces a thermal mismatch stress in addition to theexisting shrinkage stresses. When the combined stresses are less thanthe isochronous elastic limit of the adhesive material, the acceleratedstress relaxation occurs with no irreversible deformation in the bond.This effect is substantially improved with increasing the number ofthermal cycles during the TASR phase (i.e., initial) portion of thethermal cure cycle.

Although the utilization of the UV light source 60 directing radiation63 through filter 24 provides the desired initial UV curing of theadhesive bond between the filter holder and collimating lens, the filterholder can be modified, as seen in FIGS. 11 and 12, to provideadditional axial exposure ports for exposure by UV radiation from radialsource 61 (as seen in FIG. 6) to improve the dispersion of UV radiationthrough the glass bonding adhesive layer 55.

As shown in FIG. 11, a filter holder 26′ is shown, which issubstantially identical to filter holder 26 with respect to theprovision of a cylindrical gap by its lower cylindrical aperture 25′ foradjustment of the filter holder to the lens; however, the lower end offilter holder 26′ includes a plurality of apertures such aslongitudinally extending, radially inwardly projecting slots 70 spacedaround the periphery of the filter holder and communicating withcylindrical opening 25′ within the filter holder 26. Four to six slots70 have been found acceptable. Once a filter 24 is mounted in place asdescribed above in connection with filter holder 26, holder 26′ receivesepoxy as in the previously described embodiment, and the lens is raisedand adjusted with respect to filter 24 contained within filter holder26′ in the same manner as in the first embodiment. The light source 61,however, is moved around the periphery of the filter holder 26′directing UV radiation into slots 70 defining downwardly projecting,spaced apart legs 72 between such slots such that UV radiation isdithered into the cylindrical side walls of lens 22 which serves tofurther disperse the UV radiation uniformly within the annular spacecontaining bonding adhesive 55. By providing spaced radially extendingelongated slots 70 or other suitably shaped apertures extending throughthe side wall of the lower section of filter holder 26 ′, a light pathis provided for UV radiation to the inner cylindrical aperture 25′receiving the end of lens 22. In one embodiment, four slots 70 spaced at90° intervals around the lower section of holder 26′ were provided. Thisresults in improved uniform UV exposure to facilitate the UV curing ofadhesive 55. In this embodiment, it is unnecessary to expose the bondingadhesive utilizing a light source 60 through the filter since thebonding adhesive is uniformly exposed utilizing radiation from lightsource 61. Once the subassembly 10′, as shown in FIG. 12, is completed,it is assembled into the resultant three-port filter package 30′ in aconventional manner.

The above description is generally applicable to optical devices rangingfrom three-port devices to five-port devices, and to higher portdevices. The difficulty of manufacturing operational devices increaseswith the increased number of optical fibers and ports. Discussed beloware some of the features of the present invention which are directed todevices with five optical ports or more.

The uses and applications for five, six and higher port-countembodiments of the invention are many. For example, possibleconfigurations of multiple-port thin-film filters, splitters,circulators and isolators include: six-port devices that are formed fromtwo-fiber and four-fiber ferrule assemblies, eight-port devices that areformed from two four-fiber ferrule assemblies, and five-port devicesthat are formed from a single-fiber ferrule assembly and a four-fiberferrule assembly.

One important aspect of a multiple-port device is the tolerance for theposition of the optical fibers in the fiber ferrule 16. The core of anoptical fiber has a diameter of only about 9.5 μm. Consequently, a 1 μmshift or error in the position of the fiber can cause the IL to beunacceptable. Therefore, great care must be taken to ensure the totaltolerance in the positioning of the fibers. To achieve these tolerances,the fibers should be pre-selected to provide the core concentricitywithin a tolerance of preferably about 1.0 μm, and more preferably about0.5 μm, and most preferably about 0.1 μm; cladding diameter of 125 μmwithin a tolerance of preferably about 1.0 μm, and more preferably about0.5 μm, and most preferably about 0.1 μm; and the ovality tolerance ofpreferably less than about 0.8%, and more preferably about 0.4%, andmost preferably about 0.12%. Concentricity is the deviation of thecenter of the optical fiber core from the center of the fiber. Ovalityis defined as the difference between the largest and smallest diameterof the fiber divided by the average diameter of the fiber (i.e.,(D1−D2)*2/(D1+D2) where D1 and D2 are the largest and smallest diameterof the fiber). The pre-screening and selection of the fibers for one ormore of these characteristics have yielded the unexpected result ofproviding an assembly in which the fibers and other component parts canbe assembled and aligned in a manner that can be reliably repeated andmanufactured for commercial applications. Prior to the realization ofthis unexpected result, there were no commercially available opticalpackages having greater than three ports, and no commercially availablesix port packages. Regarding ferrule capillary tolerances, the simplest“square” capillary ferrule is preferably characterized by a tolerance ofthe output end of the capillary of 252 μm+/−2 μm as the distance betweentwo parallel sides and more preferably 251 μm+/−1 μm and most preferably250.5 μm+/−0.5 μm. Similar tolerances are preferred for the othercapillary shapes and configurations. Further, the tolerance of the fiberposition must be maintained throughout the manufacturing, packaging, andenvironmental conditions the device must endure. The methods andapparatus to achieve these tolerances are a subject of the presentinvention and are discussed below.

Although some prior art devices may initially achieve the desiredtolerances for the position of optical fibers, the prior art often failswhen the device is subject to stresses, strains and environmentalconditions that cause the fibers to shift sufficiently to exceed thetolerances. Causes of these stresses include: 1) viscous flow ofadhesive involving the fibers, 2) curing of the adhesives that bond thefibers to the ferrule, and 3) thermal stress due to the final packagingoperations or environmental testing conditions. During manufacture, thedevices are subject to heat such as from solder used to encase thedevices in a protective metal sleeve 32. In use, the devices are subjectto environmental conditions and must remain operational over aqualification temperature range from −40° C. to 85° C. (an industrystandard temperature range). Therefore, one aspect of the inventionrelates to a four-fiber-ferrule that satisfies the above-mentionedtolerances.

Ferrules are generally cylindrical boro-silicate or fused silicacomponents with one, two, three or more capillaries for receiving theoptical fibers. Ferrules 16 were discussed above in discussion of FIGS.2 and 3; however, the capillaries for six-port devices are preferablydifferent. The shape of the drawn capillaries and the illustrativefabricating techniques allow fibers to be not only symmetricallyseparated from the central axis of the ferrule, but be properly guidedand constrained as well. This minimizes the repositioning caused by theadhesive flow and the thermally induced change in the separationdistance between two pairs of the input and reflective fibers. Thecapillaries provide precision parallel positioning inside the ferruleand bonding of the fibers and thereby provide a reliable constraint ofthe fibers. Preferably, the fibers touch the nearest adjacent fiber orhave a gap between the fibers of not more than about 0.5 μm. This helpsto fix the position of the fibers. It is also preferred that the fibersdo not twist around each other over the first 10 to 15 mm before thefibers enter the ferrule to reduce stress and/or fiber repositioning. Anillustrative assembly process includes the following steps. The fibersare stripped of the protective coating and cleaned for a length of about5 cm of the fiber end. The fibers are dipped into adhesive (e.g.,Epo-Tek 353 ND). The stripped fiber ends are then fed through thecapillary until the fiber coatings just reach into the cone end portionof the ferrule. Additional adhesive is applied to the fibers if needed,and the adhesive is allowed to wick through the entire capillary. Anadhesive such as 353 ND adhesive with viscosity (at room temperature) ofabout 3000 cPs (centipoise), or other suitable adhesive, can be used.The predicted gaps in the capillaries shown correspond to thisviscosity. A higher viscosity adhesive (5000 to 10000 cPs) may be usedif the gaps are slightly larger. An increase in temperature wheninserting the fibers inside the capillaries decreases the viscosity ofthe adhesive. Thus, having various viscosities and temperatures, we canprovide a better positioning of the fibers and minimize theirrepositioning after cure. In general, a suitable viscosity can bedetermined using the Hagen-Poiseuille equation modeling viscous flow ina capillary with optical fibers positioned in the capillary.

The assembly is cured, an 8-degree angle is polished into the ferruleand anti-reflective coating is applied. The bond layers between thefibers and surrounding ferrule are extremely thin (preferably less thanabout 1-1.5 μm) to minimize thermal stress and movement. Variousembodiments of the ferrule capillaries of the present invention areillustrated in FIGS. 13A to 13H and FIGS. 14A to 14E.

FIG. 13A shows a cross-sectional view of a ferrule 16 with a roundedsquare or rounded rectangular capillary 130 and closely packed opticalfibers 131 a, 131 b, 131 c, and 131 d. The rounded square capillaryprovides a fixed SD, while the rounded rectangular capillary can makethe SD variable. The rounded corners and closely packed fibers make thisa good design for several reasons. The shape of the capillary along withthe closely spaced fibers 131 effectively prevent movement of the fibers131 prior to curing and also reduces thermal stress on the fibers aftercuring. The curvature of rounded corners 130 a preferably has a smallerradius than the outer surface of fibers 131. More preferably, thecorners 130 a are 90-degree angles and thus form a true square orrectangle capillary. Therefore, for purposes of this specification,“substantially rectangular” refers to a capillary cross section wherethe radius of the corners is less than or equal to the radius of theenclosed optical fibers. Gap G4 is where the fiber comes closest totouching, or actually touches, the wall of capillary 130. Gap G4 ispreferably less than about 0.5 μm, and more preferably less than about0.1 μm, and most preferably zero (i.e., the fiber touching the wall ofthe capillary). The gap G6 between the closely adjacent fibers 131 a and131 b (and also fibers 131 c and 131 d) is similarly small (i.e.,preferably less than about 1.0 μm, 0.5 μm, or zero μm). The gap G5 isalso preferably small (i.e., less than about 1.0 μm, 0.5 μm, or zero μm)however, the gap G5 between the distant adjacent fibers 131 a and 131 dmay be larger to achieve a desired SD as illustrated in the followingfigures. The closely packed fibers also provide a secondary advantage inthat only a small amount of adhesive is required in the capillary 130and therefore less thermal stress is exerted on the fibers 131 due tothe unequal coefficient of thermal expansion (CTE) between the fibersand the adhesive. Even the adhesive in the larger gap G5 has been foundto have minimal effect in causing stress or shifting of the opticalfibers due to thermal expansion and contraction. This capillary designtends to prevent shifting of the fibers and prevents rotation of thefibers due to the flow of adhesive prior to cure (e.g., fiber 131 d isunlikely to rotate to the position of fiber 131 a, and fiber 131 a isunlikely to rotate to position 131 b, etc.).

Once the fibers are affixed in the capillary 130, the selection of whichoptical fibers will form pairs (i.e., input and reflective) may be made.Generally, pairs of fibers will be positioned diagonally from oneanother. For example, referring to FIG. 13A, diagonally spaced fibers(e.g., 131 a and 131 c) may be selected for pairing. Light signalsmoving through diagonally spaced fibers may intersect at the same pointat the center of the optical filter 24. This may cause some interferencebetween signals. If signal interference is a problem, then using thecapillary designs with both fixed and variable SD designs for the fiberpairs may reduce the interference. Several capillary configurations arepossible and are discussed next.

Several other exemplary capillary designs include the dual-ovalcapillary (FIG. 13B), the clover-leaf or four-circular capillary (FIG.13C), the six-fiber rectangular capillary (FIG. 13D), the twowafer-ferrule (FIGS. 13E and 13F), the four-fiber rectangular capillary(FIG. 13J), the dual rectangular capillary (FIG. 13K), the variable dualrectangle capillary (FIG. 13L), the dual oval capillary (FIG. 13M), themixed capillary (FIG. 13N) and the alignment washer design (FIGS. 14Aand B). For simplicity, the same reference numbers are used forcorresponding features in each of the Figures.

A significant difference between the capillary designs is that some areuseful for a “fixed” separation distance design while others are usefulfor a “variable” separation distance design. For example, FIGS. 13Athrough 13D illustrate fixed SD designs (i.e. the SD cannot be changed).However, FIGS. 13E through 13H illustrate variable SD designs.Generally, the variable SD designs are used when larger separationdistances are desired.

Referring now to FIG. 13B, the shape of dual-oval capillary 132resembles two adjacent ovals and the capillary 132 encloses the opticalfibers 131. Portions of capillary 132 form a constraining arc 132 a ofapproximately 120° to 180° around fibers 131. The gap G4 between thesurface of the fibers 131 and the proximate wall of the capillary 132 ispreferably less than about 1.5 μm, and more preferably less then about1.0 μm, and most preferably less than about 0.5 μm. Similarly, the gapbetween closely adjacent fibers G6 is also preferably less then about1.5 μm, and more preferably less then 1.0 μm, and most preferably lessthen about 0.5 μm at the closest point. The gap G5 between the variablydistant adjacent fibers G5 preferably ranges from 0.5 μm to about 300 μmdepending on the position of the two oval capillaries. The diagonalpairs, such as fibers 131 a and 131 c, are formed into pairs of inputand reflective optical fibers. The dual-oval capillary may be expandedto three or even four adjacent ovals, if desired, to form multi-ovalcapillaries. However, in the multi-oval capillaries, diagonal pairs ofoptical fibers are preferable.

FIG. 13C illustrates a four-circular capillary 133 enclosing fibers 131.Portions of capillary 133 form a constraining arc 133 a of approximately180° to 240° around fibers 131. The gap G4 between the fiber and theproximate wall of the capillary is preferably less than about 1.5 μm,and more preferably less then about 1.0 μm, and most preferably lessthan about 0.5 μm. Also, the gap G6 between closely adjacent fibers issimilarly preferably less than about 1.5 μm, and more preferably lessthen about 1.0 μm, and most preferably less than about 0.5 μm.

FIG. 13D illustrates a rectangular capillary 130 enclosing six fibers131. Again, the gaps G4, G5, and G6 are preferably as small as possibleto prevent movement of the fibers. The gaps are therefore preferablyless than about 1.5 ∥m, and more preferably less then about 1.0 μm, andmost preferably less than about 0.5 μm. In this embodiment, the fibershave two separation distances. The diagonal fiber pairs (i.e., 131 a,131 c and 131 b, 131 d) have matching separation distance. However, thefiber pair, 131 e and 131 f, have a smaller separation distance. Whilethis configuration may be of less use with thin film filter assemblies,this configuration is useful for certain crystal based assemblies suchas isolators.

The ferrule and capillary designs described above are examples of fixedseparation distance capillaries. The separation distance between thefibers is fixed and cannot be changed. However, it is desirable to beable to change or vary the separation distance. For example, if a thinfilm filter has a certain preferred angle of incidence, then it isuseful to vary the separation distance of the fibers to correspond tothe desired AOI. The following ferrule and capillary designs provide amethod of achieving this desired separation distance while maintainingthe same positioning accuracy of the prior designs. Generally, thesedesigns maintain a fixed vertical separation between fibers whilevarying the horizontal (as seen in the Figures) distance. It has beenfound that the ability to vary the horizontal distance in a range offrom 5 μm to 75 μm is most useful.

One embodiment for a variable SD ferrule and capillary is illustrated inthe two-wafer capillaries shown in FIG. 13E where a cross-sectional viewof four fibers 131 (two pairs) are positioned inside of V-shapedcapillaries 134 a and 134 b formed from matching grooves in twoelongated silicon plates (wafers) 135 a and 135 b. The silicon wafersare etched with the V-grooves and accuracy of 0.5 μm is possible.Crystallographic orientation provides excellent angle reproducibility.Further, the wafers are easily mass produced using current etchingtechniques. The wafers 135 are each provided with four, preferablysymmetrical, grooves. The two center grooves (i.e. fiber grooves) areused to form capillaries 134 a and 134 b when the wafers are matedtogether. A feature of this design is that the V-shaped grooves may bepositioned as desired to achieve any required separation distancebetween the fibers 131. The adjacent fibers in each capillary 134preferably touch each other. Adhesive is applied to the gaps to securethe fibers 131 in place. Alignment grooves in wafers form two alignmentcapillaries 136 which are for aligning the wafers 135. Preferably, glassballs or rods 137 of about 300 μm diameter are inserted into alignmentcapillaries 136 of having dimensions of suitable size to contain rods137 up to about 302 μm in diameter to maintain alignment. The rods 137preferably have dimensional tolerance of 2.0 μm, and more preferablyhave a tolerance of 1.0 μm, and most preferably have a tolerance of 0.5μm. If the rods are too large, the fibers may have excess room to moverelative to their respective grooves. The glass rods, therefore, arepreferably prescreened to verify dimensional tolerances. UV-curabletacking adhesives and thermally curable structural adhesives are appliedfor providing structural integrity of the assembly. A more preferablewafer ferrule is illustrated in FIGS. 13F through 13H.

The wafers in FIG. 13F use smaller V-shaped grooves 138 for supportingthe fibers 131 and alignment rods/pins 137. The smaller V-shaped groovesprevent the wafers from coming into contact. It is thought that thisdesign will allow the fibers to touch adjacent fibers and therebyprevent movement or repositioning of the fibers 131. In this embodiment,the large V-shaped grooves (i.e. alignment grooves) 138 a support thealignment pins 137 and the smaller V-shaped grooves (i.e. fiber grooves)138 b support the fibers 131. The large V-grooves 138 a preferably are246 μm at their widest point. The smaller V-grooves 138 b are preferably120 μm at their widest point. Using this design, the V-grooves thatsupport the fibers 131 can be positioned as desired to vary theseparation distance of the fibers 131. Using known etching techniques,the V-grooves can be positioned with a tolerance of about 0.2 μm. Thisdesign is easily expanded to more fibers by merely etching moreV-grooves for more fibers. Even though the wafers do not touch, thechannels formed by the matching grooves are still referred to ascapillaries for this specification.

The aligned and bonded wafer ferrule 16 may then be cut, etched, ormachined (e.g., polished) to a polygonal or cylindrical shape or othershape as desired so that ferrule may be inserted inside a protectiveglass sleeve 14. This is illustrated in FIG. 13G. The end-face surfaceis processed the same as other ferrules, the end-face is ground to an 8°angle, polished, and coated with an anti-reflective (AR) material. Oneskilled in the art will understand from these examples that there areother similar capillary designs which will similarly support thepositioning of optical fibers with tolerances of about 0.5 μm.

Generally, over-etching of the V-grooves is not a problem. If theV-grooves are over-etched, only a uniform vertical shift in the wafersis induced. Of course, if the V-grooves are etched excessively, thefibers and alignment pins may have room to move or reposition. FIG. 13Hillustrates the relative position of fibers and alignment pins andV-grooves. The V-groove on the left easily restrains the movement of thefiber. However, the V-groove on the right side provides very littlerestraint on the fiber and is therefore less desirable.

While the wafer ferrule design has several advantages, the wafers andalignment rods can be expensive to manufacture and the process ofaligning the fibers properly into the V-grooves can be time consuming. Atechnique to reduce the disadvantages while still taking advantage ofthe high accuracy of the V-grooves will now be shown. Using this method,a convention ferrule and capillary may be used in combination withwafers to achieve a high degree of accuracy in positioning the fibers ata low cost. The process is as follows and is illustrated in FIG. 13I. Aplurality of optical fibers 131 are inserted into a ferrule 16. Thefibers 131 are sufficiently long to extend out the end of the ferrule16. Two silicon wafers are etched with V-grooves in the same manner asdiscussed above. The two wafers 139 are positioned around the fibers 131such that the fibers 131 are accurately positioned in the V-grooves asdiscussed above. The wafers 139 are clamped together with a spring clampor similar device. The fibers 131 are now accurately positioned andadhesive is applied to hold the fibers in place. Using this technique,an inexpensive ferrule with a low tolerance capillary can be made toposition fibers in a very high degree of accuracy which rivals thetwo-wafer designs discussed above.

The preferred method of applying adhesive to all capillaries includesapplying small portions of adhesive 144 a and 144 b to the fibers 131just outside of the ferrule 16 to block the flow of subsequently appliedliquid adhesive. This adhesive is cured before applying additionaladhesive. Additional adhesive 144 c is applied to the fibers and the endof the ferrule 16 and allowed to wick through the capillary 130. Theliquid adhesive is drawn through the capillary 130 presumably via theprocess of capillary action and emerges out the end of the ferrule whereit is blocked by cured adhesive 144 b. The adhesive 144 c is cured andthe wafers 139 are removed. The fibers 131 and ferrule 16 may then becut and polished as desired.

Another technique for applying adhesive to the fibers prior to insertinginto the ferrule. This technique has the advantage that the fibers areheld together by the liquid adhesive by capillary action. The liquidadhesive may be applied by dipping the fibers into the adhesive, orpreferably by applying a small amount of adhesive to the fibers.

There is another design for achieving variable separation distanceillustrated in FIG. 13J. In this design, a rectangular capillary 130supports four fibers 131. The fibers are positioned against the walls ofthe capillary 130 and therefore the separation distance is controlled bythe width of the capillary 130. The gaps, G4 and G6, are preferably lessthan about 1.5 μm, and more preferably less then about 1.0 μm, and mostpreferably less than about 0.5 μm. However, the horizontal gap G5between fibers may be as wide as desired. In other words, gap G5 is theshortest or minimum distance between the cladding of adjacent fibers 131b and 131 c.

Yet another design is the dual-rectangle capillary illustrated in FIG.13K. The capillaries 130 may be manufactured to tolerances of less than1.0 μm using currently known techniques and therefore the separationdistance between the fibers can be closely controlled. The dimensions ofthe capillaries 130 are specified to be 2.0 μm wider and taller than thedimensions of the fibers 131. The tolerance for the capillaries 130 is2.0 μm. Therefore, there is room for inserting the fibers into thecapillaries and while limiting the repositioning of the fibers.

Still yet another embodiment is illustrated in FIG. 13L. This embodimentallows variable positioning of the fibers 131 in both the horizontal andthe vertical positions as seen in the figure. This embodiment is similarto FIG. 13K in both design and tolerances. Although the design in FIG.13L can be used to achieve large separation distances between the fibers131, the fibers can more easily be repositioned within the capillaries130 due to stresses such as adhesive curing and thermal changes. Itshould be noted that some care must be taken to provide a reasonableseparation between the capillaries 130. It has been found that smallseparations lead to fractures and breaks in the glass between thecapillaries. In this embodiment, gap G6 is the shortest or minimumdistance between the surface of the cladding of the adjacent fibers 131c and 131 d.

FIG. 13M illustrates another dual capillary design similar to the designof FIG. 13K. However, in this instance, the capillaries are ovalsinstead of rectangles. The same fabrication techniques and tolerancesapply to this embodiment.

A hybrid of both fixed and variable separation distance fibers isillustrated in FIG. 13N. This hybrid design incorporates features of thevarious designs discussed above. An advantage of this design is thelarge number of fibers (for example, 8 as shown in the illustratedembodiment) that are fit into a single ferrule. However, the separationdistance for the fibers is not equal. The four fibers 131 a-131 d in themiddle capillary have a small separation distance while the outer fibers131 e-131 h have larger separation distance. In this embodiment it ispreferred that the optical fibers are paired as follows: fiber 131 awith 131 c; fiber 131 b with 131 d; fiber 131 e with 131 g; and fiber131 f with 131 h. Because of the two different separation distances,this design is generally not preferred for use with thin-film filters.This design is suitable for isolators and other optical elements whichare not sensitive to AOI.

Yet another process and apparatus for positioning optical fibers insideof a ferrule uses alignment washers to precisely position the fibers.This process is illustrated in FIGS. 14A and B. The process usesalignment washers 140 shown in FIG. 14A. Washer 140 is shown having fourapertures 141 for receiving optical fibers; however, it is easilyscalable to larger numbers of optical fibers. Alignment washer 140allows precision fiber placement into a ferrule 16 using simple andhighly manufacturable components. Photolithography technology may beused to manufacture the washers 140 with the precisely positionedapertures 141 and spacing between them. The diameter of apertures ispreferably about 126 μm which provides approximately 0.5 μm gap betweenthe fiber and the wall of the aperture. The tolerances for the locationof the apertures are also preferably less than about 1.0 μm and morepreferably less than about 0.5 μm for each pair of the input andreflective fibers. For example, the tolerance for the distance “D4”between the apertures 141 d and 141 b is preferably 0.5 μm. The same isapplicable to the distance “D5” between apertures 141 a and 141 c.However, the tolerance for the distance “D6” between adjacent aperturessuch as 141 a and 141 b is preferably less than about 1.0 μm and morepreferably less than about 0.5 μm. A photo-resistive material is used tofabricate the washers 140. Any other technique may be used to form thewasher as long as the necessary tolerances are achieved. The washers 140are used as optical fiber-guiding and constraining devices. Thecapillaries described above generally result in restricting fibermovement or shifting to less than about 0.5 μm.

Turning to FIG. 14B there is shown a cross-section view of the washers140, fibers 142, and ferrule 16. Fibers 142 are inserted through firstwasher 140 a, through ferrule 16, and through a second washer 140 b.Ferrule 16 may have a conventional cylindrical capillary 130. However,the invention may be adapted for use with most capillaries regardless ofshape. At this step of the process, it may be helpful to pre-heat theassembly to aid in the installation and precise placement of the fibers142. The assembly may then be cooled to room temperature to hold thefibers 142 in position while adhesive is applied. Washers 140 are bondedto the end-faces of ferrule 16. In the case of a ferrule having a coneportion for receiving fibers (see FIG. 5), the washer 140 is preferablybonded at the base of the cone portion where the capillary 130 meets thecone portion. The ferrule capillary 130 is filled with a liquid adhesivevia the gap created by the flat portion 143 of washer 140 and either UVcured or thermally cured. The flat portion 143 may also be used to alignthe fibers at each end of the ferrule prior to curing the adhesive. Whenboth flat portions are aligned, then the fibers are also aligned. Thecompleted assembly is processed the same as a conventional ferrule; theend-face is ground to approximately an 8° angle, polished, and an ARcoating is applied. Filter AOI and Fiber SD are discussed next.

For all of the fiber capillaries discussed above, it is important toachieve accurate SD so that the SD can be accurately matched with afilter AOI as discussed in the next section. Further, when manufacturinga fiber-ferrule having multiple pairs of fibers, it is important for SDfor all of the pairs to be approximately equal (with a tolerance ofabout 0.5 μm) since this tends to make the active alignment processeasier and more successful.

The next aspect of the invention is the relationship between the filterangle of incidence (AOI) and the optical fiber separation distance (SD).The tolerances for SD are precise so that light signals are directed towithin about 0.5 μm of the center of a desired optical fiber core. It ishelpful to define some terms prior to the general discussion of AOI andSD.

Filter AOI is well known in the art and does not require lengthyexplanation. Generally, filter AOI is useful in tuning a filter to adesired center wavelength (CWL). Each filter is characterized accordingto its CWL and AOI. The AOI value represents the desired angle ofincidence for optimal performance of the filter. For proper operationand low insertion loss, the filter should be matched to a pair ofoptical fibers having a corresponding SD.

Separation distance (SD) is defined, for purposes of this specification,as the distance between the center of the optical fiber cores of twooptical fibers. The term generally refers to SD between pairs (i.e., aninput fiber and a reflective fiber) of optical fibers. In the preferredembodiment of the invention, SD ranges from about 125 μm to about 250μm. This range of SD corresponds to an AOI range from about 2° to about3° as discussed below (see FIG. 15).

It has been found that a precise, cost effective and stable alignment ofa filter assembly 10 can be achieved by selecting components havingmatching characteristics. For example, the components of a filterassembly include the fiber ferrule 16, collimating lens 22, and filter24. The characteristics, which need to be matched, include the filterAOI, the collimating lens AOI, and the optical fiber SD. The optics ofGRIN lenses are understood and manufacturing a GRIN lens to match adesired AOI is known in the art. Matching the filter AOI and opticalfiber SD is not as easy.

Generally, matching of filter AOI and fiber SD is done by creating adatabase of measurements for the different sets of the filter chips andthe ferrules. First, filters are tested and characterized according toCWL and AOI. The measurements may be performed as follows. A filter isassembled into a filter assembly 10 or similar device so that a lightsignal may be directed onto the filter. A light signal is transmittedinto input optical fiber 18, transmitted through the collimating lens 22to filter 24. The output of filter 24 is monitored and the passfrequency or CWL of the filter is determined. The angle of the lightsignal impacting relative to the filter is adjusted until the desiredoutput signal from filter 24 is achieved. Typically for the commercialthin film filters, the resulting AOI is between about 1.8° and 3°.

While the filter 24 is at the desired AOI, the corresponding SD may bedetermined by correlation with the SD data on the ferrule sets.

Repeated testing and measurement for various filters AOI yields anaccurate database that relates filter AOI to a corresponding SD of theferrule. Those skilled in the art understand that these measurementswill vary depending on the optical characteristics of a specific designof a filter assembly and therefore should be performed on the specificdevice for best results.

After the measurements are made and the database created, tolerances maybe generated for matching input collimating assemblies with filters fora given packaging tolerance accuracy. A table as shown in FIG. 15 can begenerated showing the range of SD that may be matched to a correspondingrange of filter AOI. The components can be categorized and placed inlabeled bins so that matching parts may be done quickly and efficiently.As can be seen in FIG. 15, the range of each SD category is preferablyabout 3-4 μm. This tolerance of 3-4 μm is satisfactory for achieving anultimate tolerance of 0.5 μm since the filter 24 may be tilted a smallamount to compensate for such small variances in SD withoutsignificantly changing the CWL of the filter.

The table may also be arranged for tighter tolerances if desired. Thismay be desirable in some cases where CWL must be very precise sincechanges in filter AOI effect the CWL of the filter. Ideally, the SDs foreach pair of fibers in an input assembly will be identical or withinabout 2 μm and therefore will place the input assembly into one of thepredefined categories as shown in FIG. 15.

Once the matching input collimating assembly (4-port or multiple-port)35 and the filter 24 are selected, they may be assembled as discussedabove to form a filter assembly 10. The four-port input and dual-fiberoutput collimating assemblies will be aligned for a maximum transmittedsignal and then soldered with the outer sleeve 32 (FIG. 3) preciselyretaining the interrelationships between these collimating assemblies.The assembly of the complete multiple-port device 30 is discussed next.

Input and output collimating and filtering assemblies are affixed insideprotective sleeve 32. Output fiber-ferrule collimating assembly 35′ ismanufactured in nearly the same way as input collimating assembly 35.However, depending on the application, fewer of the fiber pigtails 38may be needed. Also, it is preferred to use an aspheric collimating lens(which may be a molded aspheric lens) instead of a GRIN lens in theoutput collimating assembly 35′. Aspheric lenses have advantages inapplication to 6 port and higher port devices as compared to GRINlenses. First, aspheric lenses have a long working distance, defined asthe distance from the front focal point to the front surface of thelens. For multiple-port devices, the input and output collimatingassemblies should have their focal points coincide in order to optimizethe insertion loss. This point should also coincide with the filtercoating surface of the filter. For multiple-port collimating assembliesthat are on the substrate side of the filter, the working distance mustbe large enough, or the filter must be thin enough, so that the focalpoint can be placed on the filter coating surface of the filter. If GRINlenses alone are used, then the filter thin films and substrate wouldneed to be very thin (on the order of 240 times the refractive index ofthe substrate, in μm). At this thinness, the filter films and substrateswould have limitations associated with film stress and also highsusceptibility to breakage, cracking, etc. during manufacturing.Aspherical lenses have working distance on the order of 2 mm whichallows a standard filter and substrate thickness of about 1.5 mm (andlarger) to be used. Therefore, a preferred configuration includes a fourfiber ferrule, a GRIN or asphere lens, a bandpass (thin film filter)coating, a substrate, an asphere, and a dual fiber ferrule. Thefollowing configuration is also possible while still optimizinginsertion loss: a four fiber ferrule, an asphere lens, a substrate, abandpass (thin film filter) coating, a GRIN or asphere, and a dual fiberferrule.

Another advantage of aspheric lenses is the flexibility in focal length.In order to keep the angle of incidence to the filter low, a longerfocal length of the lens is desirable. This is relatively easy toaccomplish with an aspheric lens. Molded asphere lenses are availablewith many different focal lengths at a low cost. For GRIN lenses, tomake the focal length longer, the index profile must change, whichrepresents a significant departure from the standard doping process. Itis difficult and costly to obtain GRIN lenses at arbitrary focallengths. All of the above makes aspherical lenses more attractive forthis application.

Preferably, the output collimating assembly 35′ is manufactured in thesame way and to the same tolerances as the input collimating assembly35. This is preferred so that the location of output optical fibers 38will match with the corresponding reflective fiber 20 in the inputcollimating assembly 35. Also, it is easier to determine the SDcharacteristic. If pairs of optical fibers are not used in the outputcollimating assembly 35′, then an estimate of the SD is made. The outputcollimating assembly 35′ is optically aligned with filter 24 bymicro-tilting, rotating, and axially adjusting the assembly 35′ formaximum transmission. This is possible because the interior dimension ofprotective sleeve 32 is substantially larger than the exteriordimensions of output assembly 35′. Micro-tilting may be achieved by amicro-tilting device grasping both the protective sleeve 32 and the endof the output assembly 35′ that extends from the protective sleeve 32.The preferred embodiment provides a gap of about 50-100 μm which issufficient to permit micro-tilting of output assembly 35′ inside ofsleeve 32. Once the active alignment of output collimating assembly 35′is complete, output collimating assembly 35′ is affixed using a solderor adhesive 33 which is inserted into the gap between the exterior ofcollimating assembly 35′ and the protective sleeve 32.

The previous discussion has related to how to manufacture multiple-portdevices such as four-fiber ferrules and six- and eight-port filteringpackages. The following discussion relates to further applications ofthese devices and additional advantages of the invention.

Turning first to FIG. 16A, there is shown a schematic diagram of afour-port filtering assembly which includes a first input fiber 160 a, afirst reflective fiber 160 b coupled to a second input fiber 160 c, anda second reflective fiber 160 d. Also illustrated are ferrule 16, lens22, and filter 24. In operation, a light signal is input through firstinput fiber 160 a, collimated by lens 22 and partially reflected byfilter 24. The reflected signal is received by first reflective fiber160 b and communicated to second input fiber 160 c. The signal is againcollimated by lens 22 and partially reflected by filter 24 and finallyreceived by second reflective fiber 160 d which can communicate thesignal to an optical communications system, network or a desireddestination. Features of this device provide enhanced performance whichis useful in optical communication systems.

First, the same filter is used to reflect the signal each time. This isan advantage over devices that required two distinct filters to performthis function. Further, the performance is enhanced because thefiltering characteristic is identical for each reflection. In devicesusing two filters, the filters typically have filtering characteristics,that are similar but not identical. Therefore, this design may result inimproved filtering characteristics such as, for example, sharper andsteeper cut-off frequencies. An example of uses for such devices is anotch filter. Typically one reflection from a thin-film notch filterwill provide 12 to 15 dB of separation. A second reflection from thesame filter will yield a separation of 24 to 30 dB. The device also hasapplication to various other shaping filters.

A second feature which improves performance is the coupling between thefirst reflective fiber 160 b and the second input fiber 160 c. Both ofthese fibers may be formed from a single, unbroken optical fiber. Thiseliminates the requirement for an optical coupling device between twoseparate fibers. Coupling devices typically have insertion lossassociated with their use. Elimination of the coupling device thereforeimproves the performance of the four-port filter 161.

Another embodiment of the four-port filter 161 is suitable for gainflattening filters commonly associated with optical amplifiers. Asillustrated in FIG. 16B, a signal is input by first input fiber 160 a,reflected by gain flattening filter 24 to first reflective fiber 160 b.The signal is amplified by amplifier 162 and communicated to secondinput fiber 160 c. The signal is again reflected by gain flatteningfilter 24 to second reflective fiber 160 d.

In another embodiment, a single filter assembly 161 may be used to gainflatten the signals from two amplifiers 162. FIG. 16C shows a schematicview of filtering assembly 161 coupled to two amplifiers 162 a and 162b. A light signal is input through fist input fiber 160 a and reflectedby gain-flattening filter 24. The reflected signal travels through firstreflective fiber 160 b to first amplifier 162 a. The amplified signaltravels back to filter assembly 161 through second input fiber 160 cwhere it is again reflected by gain-flattening filter 24 and outputthrough second reflective fiber 160 d to second amplifier 162 b.

FIG. 16D is an opto-mechanical schematic of a five-port filter 163. Theoperation of the filter is very similar to the assembly of FIG. 16;however, this five-port filter includes an output collimating assemblyfor receiving the signal transmitted through the filter 24. Filter 24may be any of a variety of thin film filters, such as, for descriptivepurposes, a narrow band-pass filter. A light signal is input by firstinput fiber 160 a, collimated by lens 22 and partially reflected byfilter 24. The selected narrow band portion of the signal is transmittedthrough the filter 24 to transmitted fiber 160 e. The reflected portionof the signal is communicated through first reflective fiber 160 b andsecond input fiber 160 c and reflected again by filter 24. The twicereflected signal is then output by second reflective fiber 160 d and theisolation from the transmitted frequency is as high as 24-30 dB.

In yet another embodiment, the filtering package is coupled to heat sinkports or terminals 165 to dissipate excess signal energy. In thisembodiment, illustrated in FIG. 16E, filter 24 may be any of a varietyof thin film-type filters such as a band-pass filter or gain-flatteningfilter. A first input signal is transmitted through first input fiber160 a, collimated by lens 22 and partially reflected and partiallytransmitted by filter 24. The transmitted portion is transmitted throughlens 34 to first transmitted fiber 160 e which is presumably coupled toa communications system. The reflected portion of the first input signalis reflected back through lens 22 to first reflective fiber 160 b whichis coupled to a first terminal 165 a. Terminals 165 are heat dissipationdevices commonly known in the art which harmlessly dissipate the wasteenergy. A similar path is followed by a second light signal that istransmitted through second input fiber 160 c. The transmitted portion ofthe signal is transmitted to second transmitted fiber 160 f and thereflected waste energy portion is channeled to a second terminal 165 bvia second reflective fiber 160 d.

FIG. 16F illustrates another embodiment with integrated waste energyheat sink ports. However, in this embodiment, the heat sink terminals165 are coupled to the transmitted fibers 160 eand 160 f. The reflectedsignals are output through first and second reflective fibers 160 b and160 d and presumably connected to a communications system.

FIG. 16G is a schematic diagram of an add/drop package using the instantinvention. Filter 24 is a thin-film band pass filter which passes lightat wavelength λ1 and reflects all other wavelengths. Lenses 22 and 34are preferably collimating GRIN lenses. A first light signal enters viainput fiber I₁ with wavelengths λ₁ . . . λ_(n). Wavelength λ₁ istransmitted to fiber T₁ and wavelengths λ₂ . . . λ_(n) are reflected tofiber R₁, and thereby one channel or wavelength is dropped orde-multiplexed from the input signal. Conversely, another signal isinput to fiber I₂ with wavelengths λ₂ . . . λ_(n) and a third signal isinput to fiber I₃ with a wavelength of λ₁. The wavelengths λ₂ . . .λ_(n) are reflected by filter 24 to fiber R₂. In addition, thewavelength λ₁ is transmitted through filter 24 to fiber R₂. Thereforefiber R₂ exits the package 171 with wavelengths λ₁ . . . λ_(n) andthereby one channel or wavelength is added or multiplexed into theoriginal signal from fiber I₂.

FIG. 16H is a schematic diagram of an eight-port package 166. Thisembodiment includes four input fibers I₁ . . . I₄ that are coupled tofour transmitted fibers T₁ . . . T₄ through an input collimating lens22, an optical element 24, and an output collimating lens 34. Theoptical element may be any of various shaping filters such as again-flattening filter or band-pass filter. However, this embodiment isparticularly well-suited to be used with a crystal element, such as anisolator, as the optical element of choice.

The final embodiment of an optical package is an eight-port add/dropdevice. FIG. 161 is a schematic diagram of the package which is capableof both adding and dropping a channel for two separate light signals.The operation is as follow. A first light signal with wavelengths λ₁ . .. λ_(n) is input through fiber I₁. Filter 24 is a band pass filter whichpasses only light of wavelength λ₁ and reflects all other wavelengths.Therefore, wavelength λ₁ is transmitted to fiber T₁ and the remainingwavelengths, λ₂ . . . λ_(n), are reflected to fiber R₁. A second signalhaving a wavelength λ₁ is input through fiber I₂. Fiber I₂ is opticallyaligned with fiber R₁ and therefore the signal is passed through filter24 and coupled to fiber R₁ and the resulting signal on fiber R₁ containswavelengths λ₁ . . . λ_(n). Thus, the original channel at wavelength λ₁and a new channel at wavelength λ₁ has been added. The same operation isaccomplished on fibers I₃, I₄, R₂, and T₂.

In yet another embodiment, a compact DWDM module is created asschematically illustrated in FIG. 17. This figure illustrates afour-channel add/drop module useful in a communications system.Concatenating four six-port filtering packages 171 (such as described inrelation to FIG. 16G) together creates the module. Beginning with thedemultiplex (i.e. drop function), the demux signal containingwavelengths λ₁ . . . λ_(n) enters the package via first input fiber I₁of package 171 a and is collimated by input lens 22. A portion of thesignal, λ₁, is transmitted through filter 24 a and transmitted out ofthe module via transmitted fiber T₁. The remaining wavelengths λ₂ . . .λ_(n) are reflected to reflective fiber R₁ and communicated to the firstinput fiber of package 171 b. Filter 24 b in package 171 b transmitswavelength λ₂ to transmitted fiber T₂ and reflects the remainingwavelengths λ₃ . . . λ_(n) to reflective fiber R₂ which communicates thesignal to the first input fiber of package 171 c. The process continuesand wavelength λ₃ is transmitted to fiber T₃ in package 171 c.Similarly, wavelength λ₄ is transmitted to fiber T₄ in package 171 d.

The DWDM module 170 also multiplexes signals. Starting with the secondinput fiber 12 of package 171 a, a signal of wavelength λ₁ istransmitted through filter 24 a to transmitted fiber T5 and coupled topackage 171 b. In package 171 b, a signal of wavelength λ₂ is similarlyinput and transmitted through filter 24 b. Filter 24 b reflectswavelength λ₁ and thereby causes both wavelengths to be multiplex andcommunicated to package 171 c. In package 171 c, wavelength λ₃ is addedto wavelengths λ₁ and λ₂. The signal containing all three wavelengths iscommunicated to package 171 d where wavelength λ₄ is added.

It will become apparent to those skilled in the art that variousmodifications to the preferred embodiment of the invention as describedherein can be made without departing from the spirit or scope of theinvention as defined by the appended claims.

The invention claimed is:
 1. An optical package comprising: an inputferrule comprising at least one capillary extending axially through saidferrule; at least two pair of optical fibers extending through said atleast one capillary, said fibers comprising a first input fiber, a firstreflected fiber, a second input fiber and a second reflected fiber, saidfibers screened for a pre-determined tolerance for a characteristicselected from the group consisting of core concentricity, ovality, anddiameter, wherein the pre-determined tolerance for ovality is equal toor less than about 0.8 percent; and an optical filter optically alignedwith said optical fibers such that a first wavelength of optical signalstransmitted through said first input fiber are reflected by said filterto said first reflected fiber and a second wavelength of optical signalstransmitted through said second input fiber are reflected by said filterto said second reflected fiber.
 2. The optical package of claim 1,further comprising an aspheric lens optically coupling said first inputfiber to said filter.
 3. The optical package of claim 1, wherein saidfilter is selected from a group consisting of a gain flattening filter,a notch filter, a band pass filter, and a shaping filter.
 4. The opticalpackage of claim 1, wherein said first reflected fiber is coupled tosaid second input fiber.
 5. The optical package of claim 4, furthercomprising an optical device coupled between said first reflected fiberand said second input fiber.
 6. The optical package of claim 5, whereinsaid optical device comprises an optical amplifier.
 7. The opticalpackage of claim 1, further comprising: an output ferrule comprising acapillary extending axially through said ferrule; and a transmittedfiber extending through said output ferrule capillary, said transmittedfiber optically coupled to said first input fiber.
 8. The opticalpackage of claim 7, further comprising an aspheric lens opticallycoupling said transmitted fiber to said filter.
 9. The optical packageof claim 7, further comprising an energy dissipating device coupled tosaid transmitted fiber and dissipating a signal communicated from saidfirst input fiber.
 10. A multiple-port optical package comprising: aninput ferrule comprising at least one capillary extending axiallythrough said ferrule; at least two pair of optical fibers extendingthrough said at least one capillary, said fibers comprising a firstinput fiber, a first reflected fiber, a second input fiber and a secondreflected fiber, said fibers screened for a pre-determined tolerance fora characteristic selected from the group consisting of coreconcentricity, ovality, and diameter, wherein the pre-determinedtolerance for core concentricity is equal to or less than about 1.0 μm;an optical filter in communication with said optical fibers such that afirst wavelength of optical signals transmitted through said first inputfiber are reflected by said filter to said first reflected fiber and asecond wavelength of optical signals transmitted through said secondinput fiber are reflected by said filter to said second reflected fiber;an output ferrule comprising at least one output capillary extendingaxially through said ferrule; and at least two output optical fibersextending through said at least one output capillary and receiving lightsignals transmitted through said filter.
 11. The multiple-port opticalpackage of claim 10, wherein said output optical fibers comprise a firstoutput fiber and a second output fiber, and wherein said first outputfiber is in optical communication with said first input fiber and saidsecond output fiber is in communication with said second input fiber.12. The multiple-port optical package of claim 10, wherein said outputoptical fibers comprise a first output fiber and a second output fiber,and wherein said first output fiber is in optical communication withsaid first reflected fiber and said second output fiber is incommunication with said second reflected fiber.
 13. The multiple-portoptical package of claim 10, wherein said output optical fibers comprisea first output fiber and a second output fiber, and wherein said firstoutput fiber is in optical communication with said first input fiber andsaid second output fiber is in communication with said second reflectedfiber.
 14. The multiple-port optical package of claim 10, wherein atleast one of said reflected fibers is coupled to a power-dissipatingdevice.
 15. The multiple-port optical package of claim 10, wherein atleast two of said reflected fibers are coupled to a power-dissipatingdevice.
 16. The multiple-port optical package of claim 10, wherein atleast one of said output optical fibers is coupled to apower-dissipating device.
 17. The multiple-port optical package of claim10, wherein at least two of said output optical fibers are coupled to apower-dissipating device.
 18. An add/drop optical module comprising:first and second six-port optical packages, each of said packagescomprising a first input fiber, a first reflected fiber, a second inputfiber, a second reflected fiber, a drop fiber, and an add fiber, saiddrop fiber optically coupled to said first input fiber and said addfiber optically coupled to said second reflected fiber; wherein saidfirst reflected fiber of said first package is coupled to said firstinput fiber of said second package; and wherein said second input fiberof said first package is coupled to said second reflected fiber of saidfirst package.